Ion implantation systems are mechanisms utilized to dope semiconductor substrates with dopants or impurities in integrated circuit manufacturing. In such systems, a dopant material is ionized and an ion beam is generated therefrom. The ion beam is directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. A typical ion implanter includes an ion source for generating the ion beam, a beamline assembly including a mass analysis apparatus for directing and/or filtering (e.g., mass resolving) the ions within the beam using magnetic fields, and a target chamber containing a workpiece to be implanted by the ion beam.
It is common for the workpiece being implanted in the ion implantation to be a semiconductor wafer having a size much larger than the size of ion beam. In most ion implantation applications, the goal of the implantation is to deliver a precisely-controlled amount of a dopant uniformly over the entire area of the surface of the workpiece or wafer. In order to achieve the uniformity of doping utilizing an ion beam having a size significantly smaller than the workpiece area, a widely used technology is a so-called hybrid scan system, where a small-sized ion beam is swept or scanned back and forth rapidly in one direction, and the workpiece is mechanically moved along the orthogonal direction of the scanned ion beam.
To achieve the uniform dose coverage to the entire area of the workpiece, the scan widths in both directions are set so that the entire part of ion beam completely leave the workpiece at the extremes of scans, often called “overscan”. That is, the scan width is greater than the sum of the size of the workpiece plus the size of the ion beam in the dimension of the scan. However, in many cases, ion beam size in both or either direction are unknown, and quite often, the scan widths are set assuming an excessively large beam size. Such an assumption is safe in terms of providing uniform dose coverage, but it lowers the beam utilization efficiency, since the beam at overscan positions does not contribute to doping on the workpiece. Further, in some implantation situations, the size of ion beam is known to affect the doping characteristics (e.g., the dose rate effect) and it would be beneficial to know the size of the ion beam prior to the doping process.